This invention relates to optical glass compositions having a refractive index of at least 1.59 while possessing relatively low dispersion (V.sub.d .gtoreq.40.5) and density (.ltoreq.2.67 g cm.sup.-3) characteristics.
Traditionally, the ophthalmic industry has produced and distributed crown glass lenses having a refractive index n.sub.d of 1.5231.+-.0.0010, a V.sub.d of 55-60, and a density of 2.5-2.6 g cm.sup.-3. More recently, the use of plastic ophthalmic lenses composed of materials such as CR 39, which have similar optical characteristics but substantially lower densities than glass, has become widespread. Very high negative diopter prescriptions utilizing these materials have a very large edge thickness, while very large positive diopter prescriptions require very large center thicknesses. This is undesirable both from a cosmetic and a practical point of view.
The introduction of Schott S-1005 ophthalmic glass (U.S. Pat. No. 3,898,093), which has a substantially higher refractive index than traditional ophthalmic crowns and a relatively low density compared to other optical quality glasses of equivalent refractive index (see Table 1 below), has permitted substantial reductions in edge and/or center thicknesses as well as significant weight reductions relative to traditional crown glasses for higher diopter prescriptions.
TABLE 1 ______________________________________ Properties of Existing Ophthalmic Materials density .sup..alpha. 20-300.degree. C. .times. Material n.sub.d V.sub.d (gcm.sup.-3) 10.sup.-7 .degree. C..sup.-1 ______________________________________ Schott S-3 1.5231 56.9 2.61 96.2 Opthalmic Crown Schott S-1002 1.7013 29.7 4.05 85.9 Opthalmic Flint Schott S-1055 1.701 31.1 2.99 101 Sovirel D00.35 1.700 34.5 3.18 Hoya LH1 1.702 40.2 2.99 98 Hoya LH1-11 1.600 40.2 2.59 CR 39 1.501 1.32 ______________________________________
This glass composition, however, exhibits one significant disadvantage; namely, its relatively high dispersion leads to chromatic aberration effects which are annoying to users of prescription lenses.
Since the introduction of S-1005, efforts have been made to develop an alternative glass with the same high refractive index and relatively low density but with lower dispersion. Sovirel D00.35 ophthalmic glasses (French Pat. No. 2,395,961) as well as the Hoya composition LH-1 (U.S. Pat. No. 4,084,978) are the principal alternative compositions which have been developed. Their properties are also listed in Table 1. The Hoya composition LH1-I exhibits substantially lower dispersion although its refractive index and density are very close to those of S-1005, whereas the Sovirel glass dispersion is between that of LH1-I and S-1005.
In a more recent development, Hoya Glass Works introduced LH1-11, an alternate high index crown glass with good dispersion characteristics but having substantially lower density (Japanese disclosure No. 8159640). This glass, while having a lower refractive index than any of the other compositions described (see Table 1), still provides substantial weight reductions in high diopter prescriptions when compared to normal ophthalmic crown glass.